OPC UA - Open Platform Communications Unified Architecture

The Open Platform Communications Unified Architecture binding provides integration with OPC UA-enabled devices and servers. From network connectivity point of view binding acts as a TCP client. The authentication can follow several ways defined in the specification. Default mode, supported out of the box, is username/password authentication.

Table 1. OPC-UA binding capability table
Device discovery	Channel discovery	Read	Write	Subscribe
No	Yes	Yes	Yes	Yes

The OPC-UA core protocol defines common services which are utilized by this binding and described below.

Device discovery

Currently, device discovery (network level discovery) is not supported. Connection to device must be defined manually.

Thing and Channel discovery

Binding use OPC UA browse service to list tags defined on the server. Based on the browse output binding will first create nodes which can be accepted through "inbox" functionality within software. Only when nodes are accepted binding will perform second browse request to fetch channels.

This is default behavior. The discovery process assumes basic mapping between Thing - these are created from OPC UA Nodes. Channels are created from scalar properties which can be mapped to atomic/primitive types.

Binding currently skip variable size structures such as arrays.

Readout principles

The subscription specification is part of official OPC UA document. Binding makes uses this protocol feature to track data. By default, binding open subscription for each linked channel to save bandwidth on busy/big servers.

The publishInterval parameter defines subscription publishing cycle. The default value is assumed to be 1000 (ms). It can be overridden at - Bridge and Thing and Channel level in fashion similar to refreshInterval used by other bindings.

Channel definitions

Binding ships several channel definitions which reflect OPC UA scalar types. Because read and write procedure is general (it is the same for various data types), configuration of channels remains fairly simple and maps 1:1 to OPC UA nodes.

Example channel definition:

dsl

Thing co7io-opcua:node:... "Test OPC UA Node" [...] { (1)
  Channels:
    Type byte : (2)
        byte-value-from-opc  (3)
        "Byte value retrieved from OPC UA"  (4)
        [ns=3, identifierType="i", identifier="1010"] (5)
}

1	All channels belong to things, hence they are wrapped in Thing section.
2	Declaration of channel type `byte` is a valid channel kind in context of OPC UA binding.
3	Unique channel identifier within thing definition (this is technical identifier).
4	Human readable label for channel.
5	Channel configuration parameters equal to `ns3;i=1010` in OPC UA addressing scheme.

As you can see above, the channel does not have anything beyond type (byte) and node identifier. Channels can be created manually, only for selected OPC UA nodes, or through discovery process.

Textual configuration

Below example is a high level description of an OPC UA enabled server with single node and single channel. Please note that both Thing and Channel config use same configuration parameters which point OPC UA node.

dsl

Bridge co7io-opcua:client:test-device "OPC UA Client connection" [host="10.10.10.2", port=4840] {

}

Thing co7io-opcua:node:test-device:node1 "Test OPC UA Node" [ns=3, identifierType="i", identifier="1000"] {
  Channels:
    Type byte : byte-value-from-opc "Byte value retrieved from OPC UA" [ns=3, identifierType="i", identifier="1010"]
}

co7io

<?xml version="1.0" encoding="utf-8" ?>
<things xmlns="http://connectorio.com/xmlns/managed/things">

  <bridge type="co7io-opcua:client" label="OPC UA Client connection">
    <id>test-device</id>
    <config>
      <host>10.10.10.2</host>
      <port>4840</port>
    </config>
  </bridge>

  <thing type="co7io-opcua:device" label="Test OPC UA Node">
    <id>node1</id>
    <bridge>co7io-opcua:client:test-device</bridge>
    <config>
      <ns>3</ns>
      <identifierType>i</identifierType>
      <idenifier>1000</idenifier>
    </config>
    <channel>
      <id>co7io-opcua:device:client1:node1:byte-value-from-opc</id>
      <type>byte</type>
      <label>Byte value retrieved from OPC UA</label>
      <config>
        <ns>3</ns>
        <identifierType>i</identifierType>
        <idenifier>1010</idenifier>
      </config>
    </channel>
  </thing>
</things>

yaml

---
things:
- kind: "Bridge"
  UID: "co7io-opcua:client:test-device"
  label: "OPC UA Client connection"
  configuration:
    host: "10.10.10.2"
    port: 4840
  things:
  - id: "node1"
    type: "co7io-opcua:node"
    label: "Test OPC UA Node"
    configuration:
      identifier: "1000"
      identifierType: "i"
      ns: 3
    channels:
    - id: "byte-value-from-opc"
      type: "byte"
      label: "Byte value retrieved from OPC UA"
      configuration:
        identifier: "1010"
        identifierType: "i"
        ns: 3